Various proposals have already been made to perform such color matching by illuminating the surface of the object or the tooth with polychromatic light, by picking up the light scattered by said object or said tooth, and by analyzing the spectrum of the light picked up, in particular to determine the spectral reflectance of the object or of the tooth.
In principle, such spectral reflectance data can provide a reliable color matching of a dental prostethis that is implanted next to the tooth on which measurement of reflectance has been performed, or color matching of series of objects reproducing the object on which the measurements have been performed. However, in practice, implementing that known technique allows high discrepancies between color of native teeth in the mouth and dental prosthesis which was found to be best matched or between an object and the reproductions of said object.
These discrepancies are due, firstly, to the attempts of color matching of slightly colored translucent objects by measuring their color in a wide range of chroma as it is used conventionally in most of the colorimetric methods and apparatus, and secondly, to illuminate a large surface of the tooth or object to be reproduced that leads, because of translucency, to light spread in a large area which causes the color averaging and to light leakage from the object which leads to lower measured brightness of the tooth or object to be reproduced.
The color of vital human teeth is a result of complex optical phenomena which include not only reflection and light scattering on the tooth surface but selective light absorption, light scattering inside the tooth and light reemission as well. The thickness of the tooth and blood supply are additional physiological factors which affect the color matching since the color of a tooth varies from gum to edge. As well a lot of teeth are spotted, e.g. they have small white spots which are in high contrast with the surrounding uniform tooth color. Therefore, it is necessary to perform color matching in different places of a front tooth surface to achieve the better total color matching. Present calorimetric and spectroscopic methods were based on the diffuse illumination of a large tooth area (more than 5 mm in diameter).
Slightly colored translucent objects such as teeth have small variations of chroma. They are classified normally as white, yellowish, reddish and brownish, but they have a lot of grades within each class which observer sees as a different hue. Therefore, it is necessary to develop a method of measurement which will be sensitive to very small changes of hue within a narrow range of colors instead of nonsensitive measuring in the full range of colors. We have found that more sensitive measurements can be accomplished by decreasing the number of measuring parameters and by using the distinctive behavior of a reflectance spectrum which most of all determines the color. This behavior consists of monotonous increase of spectral reflectance of the tooth in the visible range of wavelengths from 400 nm to 700 nm. The higher coloration of the teeth corresponds to larger difference between reflectance measured at long and short wavelengths. At the same time, the total reflectance of the tooth that is described as a tooth brightness, is proportional to the reflectance measured at green band of visible spectrum that is close to 550 nm and corresponds to maximum spectral sensitivity of human eye.